How many ASTM B117 salt spray hours should anodized and powder coated aluminum pass?

What does an ASTM B117 salt spray test actually measure on coated aluminum?

It measures how a coated specimen looks after a fixed time in salt fog, judged against criteria that the finish specification supplies. On aluminum there is no red rust to wait for, so the criterion is pits on anodize and chem film, and blistering or creep from a scribe on powder coat.

The scope of ASTM B117 states plainly that the practice does not prescribe the type of test specimen, the exposure period or the interpretation of the results. Per ASTM B117, the cabinet atomizes a 5 % sodium chloride solution at a controlled temperature, and that is all the method itself controls. Everything that turns the exposure into a pass or a fail lives in another document.

That matters more on aluminum than on steel. Engineers who learned salt spray on plated steel tend to write “no red rust” or “no base metal corrosion” on an aluminum drawing, and neither phrase has a defined meaning there. Aluminum corrodes by pitting, which shows as small white or gray spots with a tail of corrosion product, and under paint it corrodes by creeping sideways from any break in the film. An acceptance note has to name one of those failure modes and say how much of it is allowed, or the inspector at each end of the shipment will make up a different rule.

What does a passing MIL-A-8625 salt spray result really prove about your parts?

It proves that the anodizer’s line was producing a properly sealed coating during the test period, on whatever specimens the specification allowed. It does not prove that your alloy, your geometry or your specific lot would pass.

Read the process control clauses and the gap becomes obvious. Per MIL-A-8625F paragraph 4.3.3.1, process control tests run at least once a month, with no more than 35 days between tests. Per MIL-A-8625F paragraph 4.3.3.2, production parts are to be used where they can be adapted to the test; otherwise the anodizer uses panels, run with a production load whenever possible, and at the supplier’s option those panels are 2024-T3 or whichever alloy made up the largest share of that month’s work. Per paragraph 4.3.3.2.3 as rewritten by Amendment 1, corrosion is tested on undyed, sealed parts or panels, and a panel is at least 3 in wide and 10 in long. Per paragraph 4.5.3.1, the significant surface is inclined 6 degrees from vertical in the cabinet, and per paragraph 3.7.1.2, areas near edges, identification markings and electrode contact marks are excluded from the count.

So a compliant certificate of test can describe an undyed wrought panel, possibly from a different load, rather than your part. 2024-T3 is a high-copper alloy and a demanding one in the cabinet (MIL-DTL-5541F note 6.10 says as much for chem film), so a pass on it is real evidence about the process; the gap is everything that differs from the panel. Your black or colored parts were dyed, and the panel was not. Your part may be a high-silicon die casting, which anodizes to a thinner, patchier film (see anodizing die-cast aluminum). It may have sharp machined edges, which are exactly the areas the count leaves out, or blind holes the seal never fully reached. If the corrosion result matters to your design, ask for one of three things in the PO, in rising order of cost: the anodizer’s current process control report with the panel alloy stated; panels of your alloy run in the same load as your parts; or production parts from your lot tested to the same pit criteria. Seal quality can also be checked directly and faster, since per ASTM B680 an acid dissolution test measures the mass a sealed coating loses, which makes it a faster lot-release check than waiting two weeks for a cabinet. Its scope excludes hard coatings, coatings sealed only in dichromate and coatings given a hydrophobic treatment (ASTM B680 scope).

Is a higher ASTM B117 salt spray number a better specification?

No. Asking for more hours than the finish specification supports does not buy corrosion life; it buys failed first articles, retests and arguments about whose cabinet is right.

Neutral salt fog is a comparative check on a process, not a service-life figure; the zinc plating salt spray guide explains why cabinet hours do not convert to years outdoors. The mistakes that show up most often on aluminum drawings are these:

• Writing a four-figure hour count on a Type II anodize, when per MIL-A-8625F the requirement is 336 hours to a pit count. Many sealed coatings go further, but nothing obliges the finisher to prove a number the specification does not contain, and dyed or thin coatings may not reach it.
• Writing “no corrosion” or “no red rust” without a pit count, a creep limit or a blister rating.
• Leaving out whether test panels are acceptable, of which alloy, and whether they must be dyed like the parts.
• Ignoring the cure time before test. Per MIL-DTL-5541F paragraph 4.4.1, chem film specimens dry for 24 hours at 60 to 100 °F before exposure, so a lab report should show that the aging step was followed.
• Mixing ISO 9227 neutral salt spray on one document and ASTM B117 on another. The two cabinets are similar, but they are different documents, so cite one.
• Treating a salt spray pass as galvanic protection. A coated aluminum part fastened with stainless hardware fails at the joint for reasons the cabinet does not test; see galvanic corrosion between aluminum and stainless.

What should the RFQ say about salt spray, and what can Saola document?

Put the full corrosion requirement in the RFQ, not after the first lot: specification, hours, grading, specimen type and how often you want a result. Then get the test standard and duration confirmed in writing in the quote.

• The alloy and how each coated surface is produced: wrought, cast, machined or a mix.
• The finish specification and revision, type, class and seal.
• The test method, the exposure, and the grading criterion for pits, creep or blisters.
• Panels or parts, which alloy, and whether a scribe is required.
• Frequency: first article only, every lot, or a periodic process control report.
• Any surfaces to mask, which the masking for anodizing guide explains how to flag.

Saola has anodizing on aluminum and powder coating done by partner plants it manages, on parts machined at its lead CNC plant or sourced as castings and stampings, as listed on the surface finishing page; the quote names where each step is done. That page states that salt-spray results are supplied on request, that the test standard and duration are confirmed in the quote, and that Saola does not claim performance beyond what the test report shows. The quality page lists a salt-spray test result for first article and on request, with the test standard and duration confirmed in the quote, and salt-spray samples are part of the sample approval step before production. If your specification calls for a test standard or a finish outside that menu, the surface finishing page asks you to say so before tooling. For the rest of the package, see how to prepare a machining RFQ, then send the drawing through the quote form.